Ultrasonic demister



March 27, 1962 s. H. v. ASKLCJF 3,026,966

ULTRASONIC DEMISTER Filed March 9, 1959 I T I i INVENIOR .STURE' HARALD VIKING ASKLOF United States Patent @fifice Patented Mar. 27, 1962 3,026,966 ULTRASONIC DEMISTER Sture Harald Viking Askliif, Waxjo, Sweden, assignor, by

mesne assignments, to Macrosonics Corp., New Brunswick, NJ., a corporation of New Jersey Filed Mar. 9, 1959, Ser. No. 798,187 12 Claims. (Cl. 183-15) The present invention relates to a process and apparatus for the treatment of fine aerosols (i.e. the dispersion of minute solid or liquid particles in a gaseous medium) in gaseous currents. The treated particles which can be subjected to our process are spread over a large size range, from molecular dimensions (a few angstroms), up to tenths of millimetres. The principal results achieved by my method, can be either a mutual agglomeration of the microparticles constituting the aerosol, the acceleration of the dissolution of these corpuscules in an auxiliary liquid phase, an accelerated evaporation or an improvement of any chemical reaction rate, which can take place between a gas and a dispersed medium. I intend to cover and describe hereafter this new method and apparatus using it specially for the control and removal of the fine micronic particles, constituting in the main, the range of industrial aerosols. Up until today sonic and ultrasonic waves have been very often suggested for aerosol coagulation. Many authors have described different arrangements (see for instance A. E. Crawford, Ultrasonic Engineering, 155- 76, 1955) which always consist of setting up a standing waves system inside a resonance chamber. Since the Kundts tube experiments it is well known, that any light dust or powder sprinkled inside a dried tube containing vibrating gas, will be collected in heaps at the nodes of displacement, if the length of the column of gas, corresponds to an integral number of half wave lengths of the sound emitted in the tube. The Kundts tube experiments correspond to static conditions, since the treated aerosol does not move inside the resonance chamber. Thus low energy requirements only, are necessary. When dealing with industrial fumes and aerosols the particle suspension is moving at 3-10 feet per second inside the resonance chamber and the standing wave method is efiicient only if the acoustic source is very powerful. As a matter of fact, a high degree of particles agglomeration is rarely achieved when using less than 1 acoustic watt per c.f.m. treated. It is also important to note that when resonance occurs in resonance chambers, the result is not a simple sine standing wave but rather a much more complex vibration, made up of a fundamental wave superimposed by a series of harmonics. When operating at high intensity it is very difiicult to create a pure sinusoidal standing wave owing to the nonlinear distortion of the acoustic field. Therefore, in a classic acoustic resonance gas cleaner, instead of a single antinodal region, there are a series of antinodal planes becoming progressively more closely spaced as the fundamental antinodal planes are approached. This complex phenomenon makes it difficult to predict any theoretical degree of agglomeration between fine particles. This constitutes a considerable drawback for the standing waves method of agglomeration. Another disadvantage of this technique is the fact that when the dimensions of the enclosure become large with respect to the wave length, the frequency becomes less and less critical owing to the greater number of modes of vibration. Therefore, the particles or agglomerates are then no longer selectively collected in the special regions corresponding to the displacement nodes.

In the new proposed method I do not make use of the resonance phenomenon based on the standing wave system. I use the radiation pressure in a simplified man her. This radiation pressure existing in high intensity acoustic fields of progressive waves will be used with a view to modifying the particle trajectories, thus increasing the collisions between them, and impingements against obstacles (such as mesh or perforated screens) placed inside the acoustic field. Before going any further, it must be explained what the radiation pressure inside an acoustic field of progressive waves is, how it can be taken advantage of, and how it differs from the classical acoustic pressure which plays the main role in the previously mentioned standing waves agglomeration systems.

That electromagnetic waves must exert a pressure on a perfectly conducting, and therefore perfectly reflecting boundary was Maxwells deduction, from his general equations of the electromagnetic field. The existence of this pressure has been confirmed experimentally and it explains, for example, the curvature of the tail of a comet. Theory and experiments indicate that the pressure on a surface normal to the incident waves is equal to the energy density of the incident and refiectedradiation. Thus, the pressure on a perfectly reflecting surface is twice that on a perfectly absorbing surface.

The same phenomenon is observed with sound waves. Rayleigh gave for the radiation pressure against a flat obstacle the following formula:

where E is the total energy per cm. in front of the surface, P the mean radiation pressure, 6 the ratio of specific heats. Intacoustic waves, such pressure is the result of second order variations in the pressure in front of the surface upon which the wave impinges. It will be recalled that for small amplitude waves we may assume that the relation between P gas pressure, and V gas volume is a linear one. Actually, the graph is hyperbolic, not a straight line. Therefore, if we consider the situation in front of a rigid reflecting surface, as the particle layers surge towards and away from the boundary, we must mention that the increases in pressure above the undisturbed value are slightly greater than the decreases.

As an indication, it can be mentioned that the force acting against a reflector six inches in diameter equals about 50 grams for plane sound waves having a frequency of 20 kc., and an amplitude of 50 microns. If a liquid spherule is submitted to the radiation pressure of an intense acoustic field, the force acting upon the vesicle, can also be calculated by means of the following expression.

PR--" )\'E2)\ Where V and r are the volume and the radius of the small sphere, E the density of acoustic energy (in ergs/ cm. which is equal to the quotient (v being the speed of propagation of vibration, and i being the acoustic intensity). It should be stressed that this expression is applicable only if the diameter of the sphere is much smaller than the wavelength of vibration. As an example it could be mentioned that the radiation pressure of 158 db emission (E=200 ergs/ cm. F=20 kc.) acting upon a water droplet (radius 1 mm.) having a vibration amplitude of 50 microns, equals approx. 4 dynes/cmf The above mentionedenergy density E (generally expressed in ergs per cubic centimeter) is a square function of the frequency and, therefore, in the ultrasonic range it can reach very high values. Today acoustic sirens 1 3 powerful loudspeakers or air jet whistles can produce 100 to 1000 ergs/cm. i.e. amplitudes between 80 and 250 microns for a reference frequency of 10 kc. In other words the radiation pressure can be sufiicient (400 ergs/cmF) to hold a water droplet against the gravitational pull.

Before outlining, how I intend to use the radiation pressure in the present invention, it is again stressed, that my method bears no common point with the previous technique of standing waves, which was using mainly the acoustic pressure for agglomeration purpose.

The acoustic pressure given by the classical formula:

P=\/ 21716 (Where I is the sound intensity, p the density of the transmission medium, and c the velocity of the sound) is a regularly varying force which does not depend on the direction of emission while the radiation pressure P has a tensorial character and is closely dependent on the direction of emission.

In the present invention we use a sound source (loudspeaker, siren, whistle, tweeter, etc.) able to provide at least 50 acoustic watts and creating a field intensity higher than 140 db at 4" distance in a gaseous medium. This sound source, as represented in the FIGURE, overhangs a porous medium (metallic wire mesh, perforated screen, fritted porous metal filter, etc.) through which the aerosol flows towards the sound source. Taking for example the case of a wire mesh, it is easily understood, that some droplets having a higher inertia than the gas molecules, cannot follow the tortuous paths superimposed to the gas current inside the mesh filter. Most of the droplets will therefore impinge upon the wires and form a liquid coating. Since the wire is smooth it does not present a surface which tends to hold the liquid film. The liquid runs down the wire surface towards the lower part of the mesh and forms bigger droplets, which are large enough to overcome the entrainment of the gas stream and therefore fall towards the bottom of the collector. This is the mechanism to which the vesicles above 10 microns respond, provided the mesh possesses enough wires functioning as impingement targets. A mean gas speed not exceeding 15 feet per second is general when processing industrial aerosols, because the pressure drop is rapidly increased above this speed. The very fine particles ranging below 10 microns, unfortunately cannot be collected efficiently by direct impingement against an obstacle because of their very low inertia. The mechanics of suspension (see for instance E. Brun and Vasseur, Journal C.N.R.S., No. 3, 1947 and R.M.G. Boucher, C. R. Acad. Sci., 230, 1826-8, 1950) shows that such microdroplets follow more or less the streamlines, thus escaping entrapment by the mesh arrestor. The only way to hold back and collect such micro-particles in the mesh consists of applying a contra current force, which will stop their ascendant motion, or deviate their path against the wires.

A thorough theoretical survey of the particles behavior (trajectories calculations for example) can easily be undertaken, when writing the following differential equation:

if] d5 A d PJ[ P" J= -p"% f( 7)+p P9 for instance), 14 the relative velocity of the particle with respect to the fluid, p the specific mass of the fluid, 11 its kinematic viscosity, 1 the specific mass of the particle, d

the particle diameter, g the gravity field, and H a field of uniform forces acting on the particles.

In the case considered in this invention, the field H is the unidirectional radiaton pressure field, and the sign must be negative because the field is acting in the direction of the gravity pull. In other words, it is seen that the added forces of the acoustic radiation field, of particles inertia and pull of gravity, can partially or completely annul the mass forces, moving the microparticles upwards. Another important feature of my process is the fact that the radiation pressure also helps greatly to get rid of the liquid film covering the mesh screen. The high intensity field produces a kind of surface cavitation (cold boiling) which enables large drops, normally falling easily, to detach more quickly from the mesh wire and counteracts the surface tension forces at the lower surface of the mesh.

By way of example it can be mentioned that in the separation of hydrocarbon mist from gas, a simple mesh screen, (4" thick), treating an aerosol flow with a mean speed of 100" per second, leaves 0.] gal of liquid mist in the tail gases per million cubic feet of gas treated. When applying a sonic field with the following characteristics, just above the upper layer of the mesh (intensity 0.05 watt/cmP, frequency 8 kc.) the amount of liquid mist escaping from the mesh screen i twenty times lower. In other words, the radiation pressure has been able to control efiiciently the finer droplets (below 5 microns) which previously were too small to be trapped by the impingement mechanism.

As a complementary example of the efficiency of my method I can also mention the results achieved in the radio-active separation of aerosols and vapours. An aerosol of uranium trioxide spherules was formed from a solution of 0.5% natural uranium nitrate by heating (temperatures greater than 0.). Under the action of heat, droplets were dehydrated, and the nitrate radical was split off to give uranium trioxide microspheres. Using a three foot thick bed of high density wire mesh, the efiiciency obtained without the action of the radiation pressure, corresponded to an escape of one part per 10 billion parts of radioactive material. When applying high intensity sound with a supersonic whistle, acting at 4" above the mesh surface, (0.1 watt/cm? at 10 kc.) it was possible to decrease the exit loading down to one part per 200 billion parts of radio active material. This remarkable achievement was accomplished at an expense of only 2 kwh. (electric energy consumption of the compressor operating the sound source) per 1000 c.f.m. of radioactive aerosol treated. The pressure drop through the entire cleaning system, was between 1" and 2" W.G.

Apart from the above mentioned application dealing with the agglomeration and filtering of aerosols, the in' vention can also be used in distillation and extraction columns to assist in increasing the efficiency of such equipment. A similar set up, intense sound source and filtering medium at short distance can be included in one or more elements, between trays of a distillation tower, in such a manner that varying the radiation pressure, corresponds to auxiliary selective cut between the heavy and light molecules of a liquid solution. We have here above explained why at constant gas speed, an energy density increase corresponds to a greater collection of more fine vesicles, thus allowing only the passage of tiny gas molecules with a predominant content of low vapour pressure molecules. In other words, the same throughput can be achieved according to this acoustic method in a distillation process even with a considerably decreased heat input. For some special problems, a completely cold distillation unit could be designed, if the liquid treated is'originally evaporated by cavitation (ceramic transducers, piezoelectric generators or liquid whistles) at the bottom of the distillation column. The distillation of crude oil to obtain lighter fuels is one of the many applications of the hereabove described invention. It must also be understood that with very high radiation pressures (above 160 db) the invention can lead to the separation of radioactive isotopes or gas molecules, in a gaseous mixture. With the previously described arrangement (thick screen mesh or porous fitted metal plates surmounted by a powerful 6 kw. loudspeaker), I can achieve an enrichment of hydrogen molecules, on the top of a column of a series of twelve units (12 screens and 12 sound sources) type separator for low speed treatment of mixtures of carbon dioxide and hydrogen. This is easily understood when one remembers, that the lighter molecules passing through the screen are less susceptible to the radiation pressure field forces than the heavier gas molecules, which possess less kinetic energy and a shorter free path.

Always using the same principle and the same type of operational unit, it is easily understood that we can perform fundamental operations, such as the drying of a gas by spraying an hygroscopic solution (concentrated sulphuric acid, for example) in the fiow stream and underneath the acoustic arrestor. Fog droplets containing the adsorbed moisture, are held inside the mesh if the radiation pressure is high enough, and refiows downwards as previously described. Any gas dissolution problem can then be handled successfully by means of this invention, if a well known auxiliary mist can be dispersed, inside the treated gas, thus adsorbing the gas phase molecules before being collected and drained out of the purifier.

Also as a direct consequence of my method, and without departing from the spirit of the invention it is possible to utilize my system with a view of performing catalytic or any other type of chemical reactions between gas and liquid phases. A liquid containing a colloidal suspension of nickel particles can be sprayed and mixed with the reacting gases (hydrogen plus hydrocarbons) in front of an acoustic arrestor. Hydrogenation can thus be performed in an easy manner, and the resulting fine fog which contains the products of the reaction can be stopped by the radiation pressure and be recovered at the bottom of the tower, such as the one shown in the drawing. The reacting liquids either containing or not containing the catalytic elements can also be injected, or atomized, into the gas current in front of the acoustic arrestor, or directly introduced inside the mesh by capillarity. The latter arrangement will lead to a quick dispersement inside the arrestor, because of the high intensity level of vibrations produced through this medium. In the drawing there is shown one of the preferred embodiments of the invention. For the purpose of illustration I describe hereafter the function of one of the possible units, which permits applying the method within the industrial field.

The drawing represents a vertical cross-sectional view through the gas aerosol reaction or separation device showing the arrangements and details thereof. The casing structure can assume any suitable form. In the drawing it is shown as a vertical cylinder, with an inlet 1, a cylindrical inertial separator 2, a bottom outlet to, a spraying system 3 and 4, an obstacle consisting of porous layers of loosely interwoven metallic mesh 5 mounted directly below a sound source 7 which here is represented by an air jet whistle fitted with an acoustic reflector 6 such as an exponential horn or a parabolic reflector. The driving power of the sound source is conveyed to the generator by means of the pipe 8 fitted with gauge 9 and controlled by means of a valve 11.

Let us examine a liquid aerosol separation problem, using the heretofore described equipment. A contaminated gas stream of 10,000 c.f.m. containing H 80 mist at a concentration of 2 grains per cubic foot enters the separator tangentially at the inlet 1. Here a simple crude intertial separator working on the cyclonic principle auses some entrainment on the walls by means of centrifugation, especially of the larger particles. The particles thus eliminated will flow downwards through outlet 10. The remaining particles in suspension will flow through the opening in the separator 2 upwards through the spray ring 4. In some cases, as in very low concentrations of acid mist, a benefit may be obtained by adding a secondary aerosol which could be for example recycled H 80 or water, which increases the particle loading. At this point, approaching the underside of the obstacle, the particls will enter a further zone of influence of the radiation pressure field. A deceleration and change of direction, causing deviation from the aerodynamic course commences. This deceleration slows the particles to the optimum speed where the largest number of particle collisions occur, between suspended particles themselves and also between the particles and the obstacle.

The meshed obstacle acts as a diffuser for the radiation pressure field, and as soon as the gas stream carries the particle within the body of the obstacle, it is exposed to several forces such as heretofore described. These forces are, vibration, deviation from trajectory path, attraction to the obstacle and attraction between the suspended particles. The particles agglomerating with one another tend to have a greater chance of impinging as they are larger. The smaller particles cannot in any case pass beyond a certain point because of the radiation pressure being exercised upon and against them. They are thus levitated until they hit an obstacle and remain stuck. The droplets impinging are also subjected to the radiation pressure, which is directional and therefore as they are forced along the wire or thread of the mesh they agglomerate and a larger drop is formed which tends to flow downwards until it contacts and connects with another downward pointing wire.

Two forces are here being exerted, gravity and radiation pressure. Eventually the so formed large droplet will reach the limit of the mass of the obstacle, and fall free against the contra flowing gas stream, until it contacts the fioor or wall of the unit and flows out of exit 10. I know from previous cases that I can eliminate 97-99% by weight of such an aerosol by using an obstacle alone at gas speeds of 12 feet per second. In this case I have used gas speeds of '0" per second and reached an increase in impingement of aerosol and obstacle from between 97-99% to 99 100%. Thus, the capacity is increased by over 300% and the recovery in the same unit of time from 1.96 grains/cubic foot to between 1.99 and 2 grains/cubic foot. In this way I increase considerably the recovery and I also decrease the size of the equipment by the use of a very simple arrangement. No other known method can increase the capacity and the recovery with so little added energy consumption. The clean areosol free gases escape upwards beyond the sound source in a flue so provided.

It must be understood that, according to the industrial results desired, the process which is the subject of this invention can be performed with various gas volumes, at any temperatures or at multiple pressures, and that within the frame of the invention, the structure and details of the apparatus hereinabove described, the size and shape of its components (different types of mesh, size of wires or filter dimensions, of micropores of the porous medium) can be altered as can their setting (different distances between sound sources and arrestors). Some components can also be replaced by others which are similar such as replacing loudspeakers and tweeters by sonic and ultrasonic sirens and replacing parabolic deflector horns by such of spherical or exponential shape.

What I claim is:

1. The method of modifying the aerodynamic trajectories and the behavior of fine particles generally of 10 microns or less in size comprising moving a stream of gaseous medium, suspending said particles to the order of several grains in weight per cubic foot of medium in and for movement with said medium, passing said medium through a fine mesh obstacle while subjecting the particles to the radiation pressure of an acoustic field of airborne waves of an intensity of at least 140 decibels to drive the particles against the obstacle for agglomeration thereon.

2. The method of claim 1 in which the medium and particles move through the obstacle in opposition to the force of gravity while the agglomerated particles travel on the obstacle mesh under the action of gravity.

3. The method of claim 1 in which the direction of gaseous fiow is in opposition to the force of gravity and the particle paths are subjected to a filter located in the acoustic field and having passages therethrough the walls of which are eventiually contacted by the particles in their movement resulting from the action of the sonic waves.

4. The method of modifying the physical characteristics and arrangement of fine particles generally of 10 microns or less in size comprising moving a stream of gaseous medium in opposition to the force of gravity through a porous medium having sinuous passages therethrough larger than any particle dimension, suspending said particles in and for movement with said gaseous medium into and through said porous medium, introducing a liquid compatible with the particles into the porous medium and subjecting the said particles, gaseous medium and liquid to the radiation pressure of an acoustic field of intense airborne waves of an intensity of at least 140 decibels projected in opposition to the direction of movement of the particles while in said porous medium to collect the particles in said liquid on said porous medium.

5. The method of claim 4 in which said liquid is introduced in a dispersed phase into the gaseous medium for movement with it into the porous medium.

6. The method of claim 4 in which the velocity of the gaseous medium is sufficiently low to permit counterflow of the particles agglomerated in said liquid by gravity for collection.

7. A method for the use of acoustic radiation pressure of the order of at least 140 decibels exerted on particles of less than 10 microns in size delivered in suspension by a moving stream of gaseous medium in an attenuated dispersion comprising driving the particles against meshed or porous obstacles in the path of said stream having intermediate spacing larger than any particle dimension by delivering said pressure as intense airborne sound waves projected in opposition to said stream, and on the opposite side of said obstacles from the source of said stream to pass into said meshed obstacles, coalescing the particles on the obstacles and sweeping them counter current to the stream, and collecting the said coalesced particles out of the path of the stream.

8. Apparatus for effecting the separation of particles generally of 10 microns or less in diameter from a gas stream in which they are widely dispersed, comprising an elongated chamber extending substantially vertically, means to deliver upwardly through said chamber a stream of such medium at a velocity of the order of 10 to 50 feet per second containing a suspension of such particles to be agglomerated, a porous mass extending across said chamber in the path of said stream and having numerous sinuous passages therethrough sized to freely pass said particles, an acoustic source in said chamber above and directed toward said mass and constructed to deliver a progressive field of radiation pressure in said mass of at least 0.01 watts cm. intensity whereby to delay the upward movement of and deflect the paths of said particles into contact with and agglomeration on the material of said mass, the velocity of said medium being insufiicient to move the agglomerated particles against the effect of gravity and said radiation pressure, collection means for the agglomerated particles forced out of the mass at the bottom of said chamber and escape means for the clean treated gas above said acoustic source.

9. The apparatus as defined in claim 8 in which means is provided to introduce into the gaseous suspension of particles contiguous to said mass a liquid compatible with said particles to facilitate their agglomeration.

10. The method of claim 7 in which the particles are in the liquid phase and are caused to impinge on and merge with each other and wet the obstacles by deflection of their paths by said acoustic radiation pressure.

11. The method as defined in claim 1 in which the acoustic field has a frequency in the ultrasonic range.

12. The apparatus as defined in claim 8 in which said acoustic source is efiective to produce a field in the ultrasonic range.

References Cited in the file of this patent UNITED STATES PATENTS 940,076 Seaver Nov. 16, 1909 1,426,177 Garner Aug. 15, 1922 2,523,441 McKamy Sept. 26, 1950 2,583,252 Carraway Jan. 22, 1952 2,646,133 Schutt July 21, 1953 2,769,506 Abboud s Nov. 6, 1956 2,854,091 Roberts et a]. Sept. 30, 1958 

